In a conventional computer system of FIG. 1, a north bridge chip 11 and a south bridge chip 12 are used to control data flows among a microprocessor 10, a system memory 13, and a plurality of I/O devices including a graphics card 14. The microprocessor 10 accesses graphics data of the system memory 13 or outputs graphing commands to the graphics card 14 via the north bridge chip 11. The system memory 13 is usually a dynamic random access memory (DRAM). The graphics card 14 is electrically connected to the north bridge chip 11 via a PCI (Peripheral Component Interconnect) or AGP (Accelerated Graphics Port) bus, and comprises a graphics chip 141, a local memory 142 and an analog-to-digital (A/D) converter 143. The local memory 142 of the graphics card 14 is usually used as a frame buffer. Furthermore, the AGP memory 131 of the system memory 13 can be accessed by the graphics chip 141 in a directly access mode, and used as a texture buffer.
Nowadays, many electrical appliances are widely used with computers due to the amazing power of computers. So far, the image source processed by a computer system includes, for example, a TV tuner 15. The analog signals from the TV tuner 15 are firstly transmitted to the graphics card 14 and converted into digital signals by the analog-to-digital converter 143. Then, the digital signals are transmitted to the graphics chip 141 to be processed. The processed image signals are written in the frame buffer of the local memory 142, and then read out to be displayed. On the other hand, the digital image signals are written in the AGP memory 131 of the system memory 13 in a direct memory access (DMA) mode. The digital image signals stored in the AGP memory 131 of the system memory 13 are further converted into a file and stored in a non-volatile memory device such as a hard disc 16.
Since the amount of image data are huge, large memory space, memory bandwidth, bandwidth of the PCI (Peripheral Component Interconnect) or AGP (Accelerated Graphics Port) bus and even storage space of the non-volatile memory device are required for pickup and display of the image data. The tremendous image data transmission might reduce the overall performance of the computer system. Therefore, proper compression and decompression operations are desirably performed to solve these problems.